It is well known to equip switched mode power supplies with protection circuits, for example for over-voltage protection, over-current protection, low mains voltage, or short circuit protection. Power supplies comprising an integrated controller circuit are already provided with respective protection circuits.
In several countries it is now required to equip switched mode power supplies with a power factor correction (PFC), for reducing the permissible low frequency harmonic content of the waveform of the mains input voltage. For this purpose a step-up pre-converter may be used comprising an integrated controller circuit, which offers a compact and very effective PFC solution. Integrated circuits of this kind are for example the MC33260, manufactured by OnSemiconductor (http://onsemi.com), and L6562 manufactured by ST Microelectronics (www.st.com). Upconverter power supplies of this kind comprise a regulation loop for providing a constant output voltage. These integrated controllers comprise a variety of integrated protection circuits for the case of a malfunction of the power supply as described in corresponding data sheets.
In FIG. 1 a block diagram of the integrated controller circuit L6562 is shown. The operation of this controller circuit is known for persons skilled in the art, and a detailed description as well as application circuits are described in an datasheet, available via Internet or from ST Microelectronics, which is incorporated herewith by reference.
The integrated circuit L6562 in particular has an integrated over-voltage detection circuit in response to the comparator operating as an error amplifier, which is connected with is negative input to the pin 1. This over-voltage detection circuit has the following function: When the load of the PFC pre-regulator is very low, the output voltage of the power supply tends to stay steadily above the nominal value, and which cannot be handled by the regulation of the L6562. If this occurs, however, the comparator output will saturate low. Hence, when this is detected, the driver circuit of the L6562 is switched-off, and therefore also the external power transistor. This is regarded as a static overvoltage protection.
When the comparator goes back into its linear regulation region, the controller IC L6562 resumes to normal operation. As the result, in case of very low load, the L6562 will work in a burst mode, with a repetition rate, which can be very low. The overvoltage detection circuit coupled to the error amplifier provides also a dynamic overvoltage protection in case of a high load drop, as explained in the data sheet.